This invention relates to hydrocracking and more particularly to the hydrocracking of high boiling hydrocarbon materials to provide valuable lower boiling materials.
High boiling hydrocarbon materials derived from petroleum, coal or tar sand sources, usually petroleum residuum or solvent refined coal, are typically hydrocracked in ebullated (expanded) catalyst bed reactors in order to produce move valuable lower boiling materials such as transportation fuels or lubricating oils. In order to obtain a desired degree of hydrogenation for hydrocracking and hydrotreating, there are typically several ebullating bed reactors in series. As an example, see U.S. Pat. No. 4,411,768. In order to increase the capacity of such a system, it is necessary to increase the diameter of each of the reactors consistent with the higher treat gas rates to process this higher liquid capacity. Also, typically, all of the liquid and gaseous products from the first reactor are sent to the second reactor. As a result for a given treat gas rate: (1) the hydrogen partial pressure declines due to production of light hydrocarbon vapors from the cracking of the heavier liquid fractions and (2) the concentration of lighter and typically more paraffinic liquid components increases with increasing residuum conversion. This reduction in hydrogen partial pressure and increase in concentration of lighter more paraffinic constituents results in an increase in sediment formation, limiting the residuum conversion level which can be attained based on either product quality or reactor operability constraints.
The object of the present invention is to increase the conversion levels in an ebullating catalyst bed hydrogenation process with a plurality of ebullating bed reactors in series. The invention involves a stripping and separation step between the serial ebullating bed reactors to strip the liquid with hydrogen and separate the lighter components and feed only the remaining liquid to the next ebullating bed reactor.